1. Field of the Invention
The present invention relates to a refrigeration system and a method for cooling objects using the refrigeration system, and more particularly, the invention relates to a refrigeration system which operates with a primary refrigerant and a secondary refrigerant to provide a more efficient cooling system capable of cooling to cryogenic temperatures.
2. Description of the Related Art
The manufacture of semiconductor wafers involves various steps, such as plasma etching of the wafers, which require a wafer processing apparatus to be cooled. During wafer processing, the wafer is cooled by cooling a portion of the wafer processing apparatus called a susceptor which is positioned beneath the wafer. The susceptor is cooled by passing a cold fluid refrigerant through channels within the susceptor. The temperature to which the susceptor is cooled affects the rate of plasma etching or the etch rate of the wafer.
Manufacturers of semiconductors have expressed interest in being able to change the etch rate in a step-wise fashion, for example, to use a high etch rate to etch a wide area of the wafer and then change to a lower etch rate to reduce the width of the etch. To achieve this step-wise change in etch rate the temperature of the susceptor must be changed quickly. A series of step-wise temperature changes can be provided by controlling a cooling system with a control system having a preset temperature program. Cooling systems which are known for use in the semiconductor industry and which can achieve the cooling required for these step-wise changes in etch rate include very large mechanical cooling systems, typically using freon. However, these mechanical cooling systems suffer from the drawback that they tend to be too large for use in the electronics industry where space is at a premium. An additional disadvantage of freon cooling systems is their poor efficiency at low temperatures. The conventional freon cooling systems also do not accurately produce the desired temperatures at the susceptor, especially when a control system having a preset temperature program is used.